Properties of powder materials can be affected by the amount of moisture they contain. Examples of such materials include pharmaceutical, chemical, food, agricultural and other products. To extend shelf life, many powders are formulated and stored under conditions that minimize moisture content and product degradation. For example, the residual moisture content of some lyophilized drugs and biologically active compounds needs to be carefully controlled so that their activity is maintained over their shelf or storage life. In order to study product stability, product degradation and other product properties affected by the amount of moisture present in the powder, manufacturers typically generate powder samples of known moisture content, and, after storing the samples for a period of time, test their properties.
Existing techniques for preparing powder samples of known moisture content often are cumbersome and time consuming. It is common, for example, to place a powder sample in a large chamber of a known relative humidity. In such a case, the time required to reach equilibrium between the sample and the chamber environment can be extensive. Transferring samples to and from the chamber can offset the relative humidity in the chamber environment as well as the moisture content of the sample. In addition, non-uniform conditions throughout the chamber may result in moisture content uncertainties in powder samples placed at different positions in the chamber.
One common approach for generating a chamber environment at a known relative humidity relies on using saturated salt solutions. Preparing saturated salt solutions requires careful laboratory work. In addition, since, for a given temperature a given saturated salt provides only one relative humidity, to generate samples of different moisture contents requires multiple experiments using different saturated salt solutions.
Therefore, a need exists for a method of producing powder samples having a specified moisture content which overcomes or minimizes the above-referenced problems.
The present invention is directed to a method and apparatus for producing a plurality of solid samples having a uniform sorbate content.
The method includes directing a gas which includes a sorbate at a constant sorbate pressure, through a plurality of containers, each container enclosing a solid sample. The sorbate content of the solid samples equilibrates with the sorbate pressure in the gas, thereby causing the solid samples to have a uniform sorbate content.
In one embodiment, the method includes enclosing a plurality of solid samples in separate containers and directing a gas stream having a constant sorbate pressure through said containers, whereby the sorbate content of the solid samples equilibrates with the sorbate pressure in the gas stream, thereby causing the solid samples to have a uniform sorbate content.
In another embodiment, the method includes correlating the sorbate content of solid samples with the sorbate pressure of a gas, whereby the sorbate content of the solid can be determined by regulating the sorbate pressure of the gas with which the solid is in equilibrium, generating a stream of gas having a constant sorbate pressure and directing the gas stream through a plurality of containers, each container enclosing a sample of the solid. The solid samples equilibrates with the sorbate pressure of the gas stream, resulting in powders that have a uniform sorbate content corresponding to the sorbate pressure in the gas stream.
The apparatus of the invention includes a source of gas having a constant sorbate pressure, a plurality of containers for enclosing solid samples, and a manifold extending from the source of gas to the containers.
In one embodiment, the apparatus includes a main conduit for connection to a relative humidity generator and a plurality of side conduits, each conduit being suitable for connection to a container.
This invention has many advantages. For example, it provides a rapid method for preparing multiple samples of uniform sorbate content. The method of the invention generally is less cumbersome and painstaking than conventional saturated salt solution techniques. For example, the same apparatus can be employed to generate a gas stream having any sorbate content. In addition, the method and apparatus of the invention typically do not require a large chamber for equilibrating powder samples with the chamber environment. Also, a plurality of solid samples having a desired sorbate content can be prepared simultaneously. Such samples can be used, for example, in moisture stability studies to determine optimal moisture levels for product integrity. Furthermore, the invention can be employed to generate solid samples having a substantially uniform sorbate content, such as uniform water content.